Corrosion inhibiting composition for steam systems



Patented Jan. 8, 1952 UNITED STATES PATENT OFFICE CORROSION INHIBITING COMPOSITION FOR STEAM SYSTEMS No Drawing. Application June 19, 1947, Serial No. 755,732

6 Claims.

This invention relates to the treatment of water, steam, stem condensates and entrapped gases in boiler-steam-condensate systems in order to prevent and inhibit rust, corrosion, pitting and similar deteriorative efiects. The invention also relates to new and improved compositions to be employed for this purpose.

The use of a volatile alkaline amine material for protection against corrosion in steam and return condensate lines is not a new idea, but in actual practice their application has been hampered by insumcient knowledge of the best amine to use, and the proper method of application. This is particularly true in steam and condensate systems of great length or in tall buildings.

The potential protective qualities of certain amines lie in the fact that they volatilize with steam from the boiler, and then dissolve in the liquid condensate to neutralize carbonic acid. No one amine has volatilizing and condensing characteristics variable enough to meet the range of conditions encountered in the average steam and condensate system. For example, we have discovered that in steam distribution systems of great length and in tall buildings an amine such as ethylenediaminewill condense out at the points nearer the boiler and the lower levels and will not adequately protect the system against corrosion at the more distant points and higher levels. Likewise, in steam systems employing long horizontal steam condensate lines we have discovered that a single amine such as ethylenediamine will not provide corrosion protection at the far end of the lines.

The specific causes of corrosion in steam condensate systems are (1) carbon dioxide and (2) oxygen dissolved in the liquid condensate.

It is our belief that the carbonic acid formed from the carbon dioxide contacting the liquid phase or condensate in a boiler-steam-condensate system is responsible for most of the corrosion occurring in steam and condensate lines.

Steam vapor leaves the boiler uniformly mixed with any carbon dioxide present. Non-uniform distribution of the carbon dioxide begins to exist the instant that liquid droplets are formed in the steam because the carbon dioxide dissolves in the liquid. While the rate of condensation from the vapor depends on heat transfer and pressure changes, the rate of dissolving of the carbon dioxide in the condensate depends on such factors as temperatura'pressure, alkalinity, contact time, etc, and these conditions are different in the various parts of any stem-condensate system. Higher condensing rates of steam are known to increase the carbon dioxide content of the condensate, assuming equal carbon dioxide contents in the incoming steam. In the varied uses of steam as in radiators, heat exchangers and evaporators, difierent condensing rates are encountered and difierent carbonic acid concentrations are obtained. The design of the apparatus and the steam and condensate temperatures will also have some influence on the carbonic acid concentrations encountered.

One of the objects of the present invention is to provide a solution to the previously described problems.

Another object is to provide a new and improved method for selectively and successively inhibiting corrosion in steam condensate lines, especially long steam condensate lines.

Another object of the invention is to provide new and useful compositions for inhibiting corrosion in boiler-steam-condensate systems. Other objects will appear hereinafter.

In accordance with the invention it has been found that these varied carbonic acid concentrations in a steam system are neutralized much more effectively, and corrosion is thereby prevented throughout the system by the application of a mixture of compounds from the group consisting of ammonia and amines of difierent physical properties, e. g., condensing characteristics, boiling point, vapor pressure and molecular weight. Each individual amine has individual characteristics of volatility and condensation, neutralizing power, and stability of its carbonate salt. These characteristics determine its efiectiveness, and they vary with conditions of temperature and pressure. Therefore, with some amines, only certain sections of the steam-condensate system will receive adequate protection, but a mixture of the proper amines, each with the correct volatility characteristics for certain boiler pressures and heater temperatures, and having correctly balanced condensing characteristics, Will insure adequate corrosion protection in all parts of the system. In steam systems employing long horizontal steam condensate lines, the condensate gradually cools and the more volatile amines such as cyclohexylamine or ammonia become more soluble and provide neutralization of the carbonic acid.

It has been our observation that in steam distribution systems in tall buildings an amine such as ethylenediamine will condense out at the lower levels while a more volatile amine or am monia is carried to the high floor levels. 'By

pH of Condensate Compound Tested After Steam At Condensate Trap Holding Tank Ammonia 5. 7' 8. 4 Oyclohexylamine 6. 45 7. 8 Morpholine 8. 45 8.2

These difierent values for pH after the steam trap and after the condensate holding tank indicate the difference in volatility between these different amines and ammonia and their usefulness in the neutralization of carbonic acid acidity under different condensing conditions. By adding to the boiler or otherwise incorporating in the steam system blends of ammonia and/or amines, it was found possible to overcome the disadvantages and lack of proper corrosion inhibition which resulted when the ammonia or .an amine was employed individually.

The invention will be further illustrated but is not limited by the following examples in which the quantities are stated in parts by weight unless otherwise indicated.

Example I A composition was prepared by mixing together the following ingredients:

Parts by weight Ethylenediami-ne (69%) 15 Nitre cake 40 Sodium sulfite 20 Chestnut tannin 20 Dextrin 5 This composition was then briquetted in the form of briquettes weighing about one pound each.

Another composition was prepared by mixing tog-ether the following ingredients:

Parts by weight Tannin '20 Dextrin 8 Water '7 Sodium sulfite 40 Ammonium sulfate 25 4 Example II A composition was prepared by mixin together the following ingredients:

Parts by weight Morpholine nitrate 50 Sodium sulfate 27 Sodium acid pyrophosphate 15 Ammonium sulfate 5 Binder This composition was briquetted in the form of pellets about the size of a pecan and requiring about 60 to '70 to weigh one pound. These were employed in the same manner as the larger briquettes of the first example.

The invention is not limited to the use of any particular amines, but there are certain requirements which the corrosion inhibiting amine or ammonia should possess in order to be satisfactory for the purpose of the invention. and these may be given generally as follows:

1. The amine should distill or volatilize with steam in appreciable quantities in the boiler pressure range of superatmospheric pressures from 5 to 250 pounds per square inch or at higher pressures if such pressures are employed.

2. The amine should not cause foaming and carryover.

3. It should not decompose appreciably under boiler conditions of alkalinity, pH, pressure and temperature.

4. The boiling point of the amine should preferably be above 100 degrees C. so that appreciable amounts of the amine are not lost in venting or preheating of the feed water and so most of "the amine will be in the liquid phase in the portions of the system handling hot condensate. It should be pointed out that boiling point alone is not the factor which determines the relative volatility of the amine with steam, as the tendency to form stable hydrates and other factors also influence this behavior.

5. The amine should not form stable carbonic acid addition products which will not decompose to the amine and carbon dioxide at preheater temperatures.

6. The carbonic acid addition products of the amine should be reasonably water soluble.

7. The amine should have a low equivalent weight. The lower the weight of the amine for each neutralizing amino group the less the quantity of the amine required to inhibit corrosion in the steam system.

8. The amine should preferably be of a type which forms a protective film on the steam and condensate lines that will inhibit oxygen corrosion. I 9. The amine should preferably have such vapor pressure characteristics that in boiling dilute aqueous solutions little change occurs in the water to amine ratio, thus providing nearly constant alkalinity in the boiling water and the steam condensate.

As further illustrations of suitable amine mixtures, the following examples are given, although it is not intended to convey that other mixtures are not within the scope of our invention.

Example "III v Per cent Water Example IV Per cent Cyclohexylamine 6 Morpholine 12 Ethylenediamine (69%) 2 5 Water 80 EzampZcV Per cent Cyclohexylamine 2 Morpholine 14 Ethylenediamine (69%) 4 Water 80 Specific examples of suitable amines for most steam systems are: morpholine (B. P. 126-130 degrees 0.), monoethanolamine (B. P. 171 degrees C.), cyclohexylamine (B. P. 134 degrees 0.), benzylamine (B. P. 184 degrees C.) and dimethylethanolamine (B. P. 133.5 degrees C.). These amines are miscible with water in all proporin steam generation under higher pressures and 3 the corresponding temperatures.

As a general rule the compounds employed in accordance with the invention will have boiling points ranging from about 34 degrees C. (e. g, ammonia, boiling point -33.5 degrees C.) to 250 degrees C.

In the selection of the proper amine mixture for a given steam-condensate system, the main variables to consider are (1) boiler pressure, (2) heater temperature, and (3) whether slug or continuous feed is to be used. Slug feeding, as opposed to continuous feeding, is not recommended and should not be used unless absolutely unavoidable.

Referring to Examples III, IV and V, selection would be made according to the following tabulation:

The dosage of the compositions of our invention are preferably suiiicient to maintain the pH of the condensate in the range of about 6.5 to 7.5, although substantial corrosion protection has been observed in many plants with pH values as low as 6.1 or 6.3.

With the compositions of Examples III, IV or V, a starting dosage of 0.07 pounds per 1000 gallons of feed water of 1.0 grains per gallon methyl orange alkalinity is suggested for systems having 30% or less raw water make-up. This means that if the methyl orange alkalinity of the feed water were 10, the amount of amine composition of Example III (or IV or V) required per 1000 gallons of feed water would be 0.7 pound.

The ultimate dosage level is established and controlled by observation of the pH of the condensate. Treatment should be increased when pH tests are below the recommended average and decreased when pH tests are above. The amount of treatment required to raise the pH a given amount increases sharply as the pH increases. The pI-I tests should be made on samples taken from condensate lines at, or directly after, the points of potentially severe corrosion.

The invention is especially advantageous in steam systems employing long horizontal steam condensate lines or in steam distribution systems in tall buildings. The invention is applicable to steam systems wherein steam is generated at opcrating pressures from a high vacuum to rather high superatmospheric pressures.

Having thus described the invention, what we claim as new and desire to secure by Letters Patent of the United States is:

'1. A corrosion inhibiting composition for steam systems comprising cyclohexylamine and morpholine in a weight ratio within the range from about 3:1 to about 1:7.

2. A corrosion inhibiting composition for steam systems comprising cyclohexylamine and morpholine in the ratio of approximately 3:1 by weight.

3. A corrosion inhibiting composition for steam systems comprising cyclohexylamine and morpholine in the ratio of approximately 1:2 by weight.

4. A corrosion inhibiting composition for steam systems comprising cyclohexylamine and morpholine in the ratio of approximately 1:7 by weight.

5. A corrosion inhibiting composition for steam systems comprising cyclohexylamine, morpholine and ethylenediamine in the approximate weight ratios of 6:12:2.

6. A corrosion inhibiting composition for steam systems comprising cyclohexylamine, morpholine and ethylenediamine in the approximate weight ratios of 2:14:4.

RUSSELL W. LANE. WILLIAM H. THOMPSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,395,730 Reichinstein Nov. 1, 1921 1,903,287 Cox Apr. 4, 1933 2,060,138 Taylor Nov. 10, 1936 2,053,024 Dreyfus Sept. 1, 1936 2,323,369 Briggmann July 6, 1943' 2,469,284 White May 3, 1949 OTHER REFERENCES Synthetic Organic Chemicals, Booklet of Sharples Chem. Inc., Phila., 13th ed. (1942), pp. 24-27. 

1. A CORROSION INHIBITING COMPOSITION FOR STREAM SYSTEMS COMPRISING CYCLOHEXYLAMINE AND MORPHOLINE IN A WEIGHT RATIO WITHIN THE RANGE FROM ABOUT 3:1 TO ABOUT 1:7. 